1. Field of the Invention
The present invention relates to a memory cartridge. More specifically, the present invention relates to a memory cartridge which can be attached to and detached from a data processing unit, such as in a video game apparatus connected to a conventional television receiver.
2. Description of the Prior Art
The following are examples of data processing units in which an image is displayed on a television receiver: a video game machine called "Family Computer" (trade mark) or "Nintendo Entertainment System" (trade mark), which is manufactured and sold by the applicant of the present invention and a personal computer called "MSX" (trade mark). These data processing units are enabled by receiving an external memory cartridge containing an educational or game software program that has been written in advance and stored in the cartridge memory. A read-only memory (ROM) for storing program data and character data for display are contained in the memory cartridge.
For memory cartridges used in the above-described manner, recently, memory size has been increased to a larger capacity, for example, 1M bits or 4M bits. In the case where such a large-capacity memory is employed, since address space accessible from a central processing unit in the data processing unit is limited, a so-called "bank switching" technique has been employed.
U.S. Pat. No. 4,432,067 is a method for expanding memory capacity while the number of address lines, or the address space, connected to the data processing unit, is maintained at a predetermined number. U.S. Pat. No. 4,432,067 discloses a memory cartridge having an address decoder, a latch circuit, a supplemental memory chip and a signal line for a memory chip.
In U.S. Pat. No. 4,432,067 the address decoder detects when the address data is a specific address that requires change-over (i.e., selection) of another memory chip. Then the memory chip is switched by the latch circuit and the signal line. This means that the change-over or switching of the memory chip is controlled by a dedicated hardware circuit and is performed only on a memory chip to memory chip basis. Therefore, it is impossible to switch a bank of an arbitrary memory size. Furthermore, since only that switching determined by the hardware circuit can be executed, there are many restrictions in designing a computer game or other program.